1. Technical Field
This invention relates to analog circuits, and more particularly concerns a method and apparatus for amplifying signals.
2. Description of the Related Art
Analog circuits are often used in a broad range of signal-processing applications such as analog-to-digital converters. Such converters generally use a plurality of amplifiers and resistors to produce a digital output in response to an analog input from physical motion detectors or electrical voltages. While the amplifiers used with such analog circuits may be of different types, several advantages associated with metal-oxide-semiconductor-field-effect-transistors (MOSFET) make MOSFETs potentially very useful in analog circuits. For example, MOSFETs typically have virtually infinite input resistance and zero offset voltage, both of which are important in terms of analog design. However, because MOSFETs typically have a relatively low open-loop gain and relatively low transconductance, bipolar junction transistors were generally preferred over MOSFETs for use in analog circuits.
Several methods for increasing the output resistance of transistors used in analog circuits have been developed. For example, in Csanky, "Combing FET's for Higher Gains", Electronic Design, Sept. 27, 1963 at p. 36, the author discloses a circuit in which the source of a first junction field effect transistor ("JFET") is connected to the drain of a second JFET, while the gate of the first JFET is connected to the source of the second JFET. By combining the JFETs in this manner, the output resistance of the pair of JFETs is greater than the output resistance of each of the JFETs individually. Because open-loop gain is a function of the transconductance of a circuit multiplied by the output resistance, the open-loop gain of the JFETs collectively also was greater than each JFET individually. See also Csanky, U.S. Pat. No. 3,271,633. Another method for increasing the output resistance of MOSFETs is described in Grey et al., "MOS Operational Amplifier--A Tutorial Overview", IEEE Journal of Solid State Circuits, v. SC-17, n. 6, December 1982. In this reference, a cascode circuit using four MOSFETs is disclosed in which the output resistance and hence the open-loop gain is increased.
The references discussed above well illustrate the void in the art associated with amplifiers used in analog circuits. Methods which used MOSFETs generally required a relatively large number of components to achieve the desired gain. In addition, the methods which used JFET suffered from the disadvantages of having relatively small increase in output resistance as well as having low input resistance. For example, the output resistance of JFET circuits using the method described above was typically 50 Mohms.